Ceramic fiber heater

Abstract: A ceramic fiber heater wherein a plurality of blocks (3a, 3b ---) each of which is formed by a plurality of heater elements (1a, 1b ---) arranged side by side on a base of insulation material and connected to one another to form a series circuit, said series circuit being connected across an electric power source, and adjacent ends (4a, 4b) of the series circuits in adjacent blocks (3a, 3b) being connected to the same terminal of said electric power source.

Description: This invention relates to a ceramic fiber heater, and more particularly to an improvement of a ceramic fiber heater having a plurality of heater elements. GB-A-2 132 459 discloses a ceramic conductive fiber heater comprising a plurality of heater elements. U.S. Patent No. 4,575,619 discloses a ceramic fiber heater having a plurality of serpentine heater elements arranged side by side. Fig. 3 is a plane view of a conventional ceramic fiber heater having a plurality of serpentine heater elements to be connected across a single-phase electric power source. In Fig. 3, reference numerals 1a, 1b, --- designate serpentine heater elements, respectively. In such a ceramic fiber heater, four heater elements 1a∼1d, for example, are arranged side by side on a base made of an insulation material to form one block 3a of the heater elements. In said one block 3a, one end 4a of a first heater element 1a is connected to one of terminals of a single-phase electric power source of 200 V, for example, and the other end of said first heater element 1a is connected through a connecting line 2 with one end of a second heater element 1b, adjacent to said other end of said first heater element 1a. Similarly, the other end of the second heater element 1b is connected through a connecting line 2 with one end of a third heater element 1c, and the other end of the third heater element 1c is connected through a connecting line 2 with one end of a fourth heater element 1d, so that the first to fourth heater elements 1a∼1d form a series circuit. Both terminals 4a of said series circuit is connected across the single-phase electric power source of 200 V. A block 3b similar to said block 3a is arranged adjacent to said block 3a on said base. Fig. 4 shows a plane view of a conventional ceramic fiber heater to be connected across a three-phase electric power source of 200 V, for example. In this ceramic fiber heater, as like as the embodiment shown in Fig. 3, four heater elements 1a∼ 1d, for example, are arranged side by side on a base made of insulation material to form one block 3a. In said one block 3a, the four heater elements 1a∼1d are connected in series. Blocks 3b and 3c each similar to said block 3a are arranged side by side on the base, and each series circuit of each block 3a∼3c is applied with a phase voltage of the three-phase electric power source of 200 V. As shown in Fig. 3, both ends 4a of the series circuit in the block 3a is connected across the single-phase electric power source of 200 V. With respect to the block 3b, similarly, both ends 4b of the series circuit is connected across the single-phase electric power source of 200 V. Accordingly, a voltage difference of 200 V is formed between one end 4a of the series circuit in the block 3a and one end 4b of the series circuit in the block 3b, adjacent to said one end 4a. Normally, a distance between said ends 4a and 4b is short, such a several millimeters and the insulation resistance or specific resistance of the ceramic fiber base made of alumina silica, etc. is reduced remarkably at a temperature higher than about 1,000°C, so that a leakage current flows through the insulation material between the two ends 4a and 4b and the leakage current becomes large due to the voltage applied between said two ends 4a and 4b, and the heater element is liable to be burned and damaged. Specifically, in case of mullite (compound of alumina and silica), for example, at 900°C, the specific resistance is 2×10 5 (Ω · cm), at 1000°C, the specific resistance reduces to 5.5× 10 4 Ω cm, at 1100°C, the specific resistance reduces to 1.2× 10 4 Ω cm, and at 1200°C, the specific resistance reduces to 2.5× 10 3 Ω cm. A voltage of the power source is applied between said adjacent ends 4a and 4b of the series circuits, so that a leakage current flows through the electrically insulation material therebetween at a higher temperature and accordingly the temperature of said material portion is elevated. As a result, the insulation resistance of said material portion is reduced, so that the leakage current is increased. Such phenomenon is repeated and accordingly the temperature of said material portion is elevated abnormally, so that the heater element is burned and damaged. In case that the three-phase electric power source is used, as shown in Fig. 4, a voltage difference of about 200 V is formed between one end 5a of the series circuit in the block 3a and one end 5b of the series circuit in the block 3b, adjacent to said one end 5a or between the other end 5b of the series circuit in the block 3b and one end 5c of the series circuit in the block 3c, adjacent to sid other end 5b, so that similar defects and problems are arisen. An object of the present invention is to obviate the above defects. The above object can be attained by a ceramic fiber heater comprising a plurality of circuits each of which is formed by connecting in series a plurality of heater elements arranged side by side on a ceramic base of an insulation material and each circuit being connected across an electric power source, wherein adjacent two circuits are arranged side by side on said base so that the last end of the last heater element of one circuit is adjacent to the first end of the first heater element of the succesive circuit, and that said last and first adjacent ends of said adjacent circuits are connected to the same terminal of the electric power source. In the ceramic fiber heater according to the present invention, the same electric potential is applied to said adjacent ends of said adjacent series circuits. A current control element, such as a relay, SSR, or thyristor can be inserted between said terminal of the electric power source and said adjacent one end of the adjacent one series circuit to control the series circuit. Fig. 1 is a view explaining a ceramic fiber heater to be connected across a single-phase electric power source according to the present invention; Fig. 2 is a view explaining a ceramic fiber heater to be connected across a three-phase electric power source according to the present invention; Fig. 3 is a view explaining a conventional ceramic fiber heater to be connected across a single-phase electric power source; and Fig. 4 is a view explaining a conventional ceramic fiber heater to be connected across a three-phase electric power source. Now, an embodiment of this invention in case that a single-phase electric power source is used will be described by referring to Fig. 1. According to the present invention, one end 4a of a series circuit in a block 3a and one end 4b of a series circuit in a block 3b, adjacent to said one end 4a are connected to the same one terminal of the electric power source. The other end 4a of the series circuit in the block 3a and the other end of the series circuit in the block 3b are connected to the same other terminal of the electric power source. Other embodiment of this invention in case that a three-phase electric power source is used will be explained with reference to Fig. 2. In this embodiment, one end 5a of a series circuit in a block 3a and one end 5b of a series circuit in a block 3b, adjacent to said one end 5a are connected to the same one terminal of the three-phase electric power source. Similarly, the other end 5b of the series circuit in the block 3b and one end 5c of a series circuit in a block 3c, adjacent to said other end 5b are connected to the same another terminal of the three-phase electric power source. Other end 5a of the series circuit in the block 3a and the other end 5c of the series circuit of the block 3c are connected to the same other terminal of the three-phase electric power source. As described above, the ends adjacent to each other of the series circuits in the blocks adjacent to each other have the same electric potential and therefore the voltage difference between the adjacent ends of the series circuits in the adjacent blocks becomes zero, so that the defects of the conventional ceramic fiber heater can be obviated. It is appreciated that the maximum voltage difference between adjacent heater elements is low, such as 100 V in case of Fig. 1 or Fig. 2 and the maximum voltage difference can be reduced by increasing the number of the heater elements forming a series circuit. Further, in the present invention, helical heater element can be used instead of the serpentine heater element. It is preferable to control each heater block by inserting a current control element, such as a relay, SSR, or thyristor between the adjacent one end of the series circuit and one terminal of the electric power source at X, Y or Z portion shown in Fig. 1. In the present invention, as the heater element, nichrome, iron-chrome-aluminum alloy or the like in the sape of a wire, a strip or the like can be used. As stated above, in the present invention, no voltage difference is formed between the adjacent ends of the adjacent series circuits. Accordingly, the abnormal current is prevented from flowing between said adjacent ends of the adjacent series circuits, so that the service life of the ceramic fiber heater element can be prolonged. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A ceramic fiber heater comprising a plurality of circuits (3a, 3b ---) each of which is formed by connecting in series a plurality of heater elements (1a, 1b ---) arranged side by side on a ceramic base of an insulation material and each circuit being connected across an electric power source, wherein adjacent two circuits (3a, 3b) are arranged side by side on said base so that the last end (4a) of the last heater element of one circuit (3a) is adjacent to the first end (4b) of the first heater element of the succesive circuit (3b), and that said last and first adjacent ends (4a, 4b) of said adjacent circuits (3a, 3b) are connected to the same terminal of the electric power source. The ceramic fiber heater of claim 1 wherein a current control element is inserted between said adjacent first end (4b) of the succesive circuit (3b) and said terminal of the electric power source. The ceramic fiber heater of claim 2 wherein said current control element is one of a relay, SSR and thyristor. The ceramic fiber heater of claim 1 or claim 2 wherein said heater element (1a, 1b ---) is serpentine. The ceramic fiber heater of claim 1 or claim 2 wherein said heater element (1a, 1b ---) is helical.

Inventors: NAGANO SHIGETOSHI, WATANABE TORU, NAGANO, SHIGETOSHI, WATANABE, TORU, NAGANO, SHIGETOSHI, C/O SAKAGUCHI DENNETSU K. K., WATANABE, TORU, C/O SAKAGUCHI DENNETSU K. K.
Applicants: SAKAGUCHI DENNETSU KABUSHIKI K, SAKAGUCHI DENNETSU KABUSHIKI KAISHA
Application year: 1994
Main classification: H05B3
IPC codes: H05B3